Silent Spring: A Sequel?

Climate change already is affecting the range and behavior of many North American birds; some scientists fear these shifts are just a hint of what's to come

12-01-2002
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Les Line

BY THE LATE 21ST CENTURY, says scientist Jeff Price, the distribution of North American birds will almost certainly look nothing like the species’ range maps found in today’s popular field guides. Some startling possibilities: State birds such as the Baltimore oriole (Maryland) and black-capped chickadee (Massachusetts) will have vanished from their official residences; there will be painted buntings in southern Minnesota but no bobolinks; the golden-cheeked warbler of Texas Hill Country will be extinct, along with other endangered songbirds whose habitat is sharply limited; and Neotropical migrants like the Cape May warbler will have moved farther north, leaving the southern boreal forest more vulnerable to devastating outbreaks of spruce budworms, which the birds now feast on during the breeding season.

Price, director of climate change impact studies for the American Bird Conservancy, is coauthor with NWF researcher Patricia Glick of a recent report, The Birdwatcher’s Guide to Global Warming. The report’s projections of avifaunal chaos are based on his computer models, which assume a doubling of atmospheric concentrations of carbon dioxide (CO2) over levels found in preindustrial times, as well as substantial increases in other heat-trapping greenhouse gases. According to the world’s leading climatologists, such changes may happen sometime over the next 50 to 100 years. And although the past decade was the warmest on record so far, scientists also project average global temperatures will rise another 2.5 to 10.4 degrees F by the year 2100. “That’s ten times faster than the sustained rate of temperature change since the last ice age,” says Price.

SILENT SUMMER? Summer ranges of the Baltimore oriole (above), American goldfinch and Carolina chickadee are likely to change as climate warms. Some states may lose these songbirds altogether; in others, their distribution could shrink.

Already, there are signs that birdlife is responding to climate change. In an analysis of three decades of data published in Nature last spring, researchers reported that a dozen British bird species have shifted their ranges an average of 12 miles north over the past 20 years. In North America, summer distributions of many Neotropical migrants also are shifting northward. The biggest change so far involves the golden-winged warbler, a strikingly patterned denizen of brushy clearings. The center of its range has moved nearly 100 miles north over the past two decades, says Price.

Warming also has had a measurable impact on the timing of such seasonal events as migration and breeding. According to the Nature study, numerous European and North American bird species are migrating an average of nine days earlier and breeding ten days sooner than they did 30 years ago. A long-term study on Michigan’s Upper Peninsula found that 15 spring migrants, including the rose-breasted grosbeak and black-throated blue warbler, were arriving up to 21 days earlier in 1994 than they had in 1965. Price and his colleagues say such changes are merely hints of what to expect as temperatures continue to rise.

To map the potential summer distribution of North American songbirds in the future, Price developed large-scale statistical models of how climate variables such as average seasonal temperature and precipitation determine a species’ present range. He then applied to these models Canadian Climate Center projections of conditions that may exist when atmospheric CO2 levels double. His results, summarized in the report, are both dramatic and troubling.

Price found that Great Lakes states, for example, may lose 53 percent of the Neotropical migrant songbirds that presently nest in the region. Other projected losses include 45 percent fewer Neotropical migrants in the Mid-Atlantic region, 44 percent fewer in the northern Great Plains and 32 percent fewer in the Pacific Northwest. Some losses could be offset by new species moving into an area. The Southwest may gain “some great new Mexican birds,” Price notes. Yet overall avifaunal diversity is expected to decline.

Here Today, Where Tomorrow?To determine how the summer distributions of birds might change in the future, Jeff Price of the American Bird Conservancy developed large-scale statistical models of the association between current bird distributions and climate variables such as average seasonal temperature and precipitation. After confirming that the models of predicted bird distributions matched maps of actual distributions, he used the Canadian Climate Center’s General Circulation Model to project how conditions in North America may change if atmospheric CO2 concentrations double from preindustrial times (which is likely sometime within the next 50 to 100 years). His results for the Baltimore oriole and American goldfinch appear below.

Baltimore Oriole

American Goldfinch

Meanwhile, warming also may affect songbird habitat. Because native forests are adapted to local climates, many trees acclimated to cool environments are likely to shift northward. In New England, for example, southern oaks and hickories may replace today’s mix of maple, birch and beech trees. Subalpine spruce-fir forests could die out as mountain habitats warm, and subalpine meadows in the Rocky Mountains will probably disappear. As the Southwest becomes moister, desert landscapes could be transformed into grasslands and shrublands.

The impact of these new distributions—of both birds and the ecosystems they inhabit—could be complicated by continued changes in the animals’ migration and breeding behavior. For some species, climate change may actually turn out to be a good thing, at least in the short term.

Consider Virginia’s prothonotary warbler. In swamp forests along the James River, Virginia Commonwealth University biologist Charlie Blem has been inventorying this species for nearly two decades. He reports that the birds, golden orange beacons in a gloomy forest, have been arriving from their Caribbean and South American wintering grounds an average of one day earlier each year since 1987. (Last spring the first male warblers were sighted on April 3 compared with April 21–23 in the project’s early years.) During the same period, the average springtime temperature at Richmond’s airport has increased nearly two degrees. “I predict that if this trend keeps up, the species will become a year-round resident of North America,” Blem says.

The warblers also are thriving. Blem has erected 316 nest boxes for the birds along a 19-mile swampland trail that takes his team of students and volunteers a week to cover by canoe—counting eggs, banding nestlings and capturing adults. Though the species has been declining in many of its Southeast strongholds, James River warblers are “nesting earlier, incubating more eggs, raising as many as three broods and surviving longer,” says Blem.

The downside of early migration and reproduction, of course, is that a species’ breeding cycle could get out of sync with its food supply. “As a result,” says Price, “early birds may not get the worm.” At the Rocky Mountain Biological Laboratory in Colorado, for example, robins have been arriving an average of two weeks earlier in spring than they did in the late 1970s, apparently in response to warmer temperatures at their low-altitude wintering grounds. But when robins reach the mountaintop now, it is still winter, and the birds must wait longer for snow to melt before they can feed. This puts the birds and their offspring at greater risk of starvation.

NWF PriorityState Birds Taking FlightWhen the Massachusetts legislature named the black-capped chickadee as the state’s official bird in 1941, lawmakers did not consider the possibility that the species might someday no longer live there. But sadly, the ranges of chickadees and other species may be so altered by global warming that the birds could disappear from the states where they are honored.

These predictions are among those made by scientists in the recent report A Birdwatchers Guide to Climate Change. Other birds that may vanish from their states include the Baltimore oriole (Maryland), purple finch (New Hampshire), brown thrasher (Georgia) and American goldfinch (Iowa). To find out how birds in your state are faring, see our global warming page.

Other ecosystem connections are threatened by warming as well. According to biologist Terry Root of the Stanford University Center for Environmental Science and Policy, “well-balanced bird communities as we know them will likely be torn apart. As species move, they may have to deal with different prey, predators and competitors as well as habitats that are less than ideal.”

Many birds, Root reminds us, are linked to specific vegetation—and it could take decades or centuries for plants to respond to global warming. “The endangered red-cockaded woodpecker of the Southeast needs mature pine forest, habitat that is already scarce. If the species has to shift its range northward to stay in a cooler environment, can it adapt to use other trees?” she asks.

If an ecosystem, in turn, loses a bird that helps control insect pests, the results could be catastrophic, for humans as well as other species. In the boreal forests of eastern North America, for instance, nesting wood warblers are important predators of the eastern spruce budworm, which defoliates millions of acres of timberland every year. Without the birds, those losses would likely be far greater. Under normal conditions, warblers consume up to 84 percent of the budworm’s larvae and pupae.

Price’s models, however, project that with a doubling of CO2, three of the most significant budworm predators—Tennessee, Cape May and blackburnian warblers—may disappear from spruce-fir forests south of Hudson Bay. This “decoupling of predator and prey,” he warns, could lead to large-scale pesticide use if budworm outbreaks get severe.

Similar problems could occur in the West, where savannah sparrows, sage thrashers and other species that help control rangeland grasshopper populations are expected to move north. “A single pair of savannah sparrows raising their young consumes an estimated 149,000 grasshoppers over the breeding season,” says Price. “Unless all of the components of this ecosystem—grasslands, insects and birds—change at the same time, an unlikely prospect, we’re looking at more grasshopper outbreaks in the future.”

NATURAL PEST CONTROL: The Cape May warbler (left) is a major predator of the eastern spruce budworm. If global warming shifts the range of this and other warblers, boreal forests of eastern North America may lose millions more acres to the insect pest.

For birds, the most devastating consequence of global warming would be loss of entire habitats on which species depend—and, unfortunately, scientists say such losses are a very real possibility. On the East Coast, for example, a rising Atlantic Ocean would inundate coastal marshes and beaches critical to the survival of 150 species of shorebirds, waterfowl and other birds. Each spring, the shores of Delaware Bay alone provide a critical food bonanza for about a million migratory shorebirds that feast on the eggs of horseshoe crabs that spawn there. Drops in the bay’s horseshoe crab population, caused in part by habitat loss, have already resulted in declines of up to 50 percent among some of these species, including sanderlings, red knots and ruddy turnstones.

Droughts across the continent’s northern prairies, meanwhile—also projected by climate models—could dry up tens of thousands of “prairie potholes,” vast seasonal wetlands that are crucial to millions of waterfowl throughout the breeding season. Loss of these wetlands would decimate populations of mallards, blue-winged teal, gadwall and other ducks.

What would be most tragic about such losses is that actions needed to slow global warming already are well known: Reduce emissions of CO2 and other greenhouse gases from fossil fuel-burning power plants, factories and automobiles. Yet the world’s industrial nations—most notably the United States, the planet’s biggest polluter—have so far done very little to solve the problem. In a recently released report, U.S. Climate Action 2002, the Bush administration acknowledged that continued build up of greenhouse gases would damage wildlife and ecosystems, yet it did not endorse any meaningful steps to reduce pollution.The report pleased neither industry spokesmen, who claimed its message was too dire, nor environmentalists. “The administration now admits that global warming will change America’s most unique wild places and wildlife forever,” says NWF President Mark Van Putten. “How can it acknowledge a disaster in the making and then refuse to solve the problem, especially when solutions are so clear?”

New York-based Field Editor Les Line wrote about grosbeaks in the August/September issue.

Can Birds Adapt?Early birds beware: Breed too soon, and the worms needed to feed hungry hatchlings may be nowhere to be found. Yet such loss of synchronization with food sources is exactly what many scientists fear will happen more frequently as birds migrate and breed earlier in the year in response to warming climate. Even with only moderate warming, some species are already arriving at breeding territories before food is available for their offspring.

Still unknown is the extent to which birds can learn from such mistakes, adjusting the timing of migration or reproduction the following year to better coincide with food abundance. Recent research published in Science provides encouraging hints that at least some species can make adjustments. In Heteren, the Netherlands, biologists at the Center for Terrestrial Ecology monitored breeding pairs of blue tits—small birds similar to chickadees—in outdoor nest boxes over a period of two years. Inexperienced at the start of the experiment, all birds tended to breed slightly later than the region’s peak caterpillar population.In the study’s first year, scientists supplemented the diets of half the breeding pairs with caterpillars and mealworms. The following season, those pairs laid eggs on about the same date they had the year before. Birds that had not been given extra food, on the other hand, bred earlier the second year, and their eggs hatched closer to when caterpillars naturally were abundant. The results suggest that birds such as blue tits may be able to adapt to some of the ecological disruptions caused by global warming.

But Terry Root, a biologist at Stanford University, warns that “not all birds are going to exhibit such plastic breeding behavior.” The least flexible species, in fact, may turn out to be those that also are most vulnerable. Worrisome support for that hypothesis comes from a recent study of red-cockaded woodpeckers in North Carolina. Analyzing nearly two decades of data on more than 200 pairs of these endangered birds, a team led by biologist Karin Schiegg of the University of Zurich found that females on the whole have been laying their eggs increasingly earlier in response to warming climate, as have many temperate zone species. Notable exceptions to that rule, however, were woodpecker pairs that included at least one inbred partner, which also produced fewer offspring than pairs with no inbred birds. Because imperiled animals often live in small, isolated populations that become inbred, “climate change poses a previously unknown threat that may hasten the decline of endangered species,” concludes Schiegg.

NWF PriorityCombating GlobalWarmingGlobal climate change poses risks to far more than birds. Worldwide, animal and plant species are threatened, as are the ecosystems they inhabit, from coral reefs and sandy beaches to forest, desert and alpine habitats. NWF’s GlobalWarming and Wildlife Program documents these risks—and promotes ways that both government and citizens can contribute to solutions. Recent efforts include publication of Wildlife Responses to Climate Change: North American Case Studies and Beneath the Hot Air, a report that critiques the current administration’s response to global warming. For more information about the program, including a list of steps each of us can take to reduce greenhouse gas pollution, see our global warming page.

Web ExclusiveAcid Rain and SongbirdsAs if global warming weren't a big enough problem for birds, recent research suggests that acid precipitation—also caused by air pollution—may be contributing to the decline of North American songbirds as well.

In a study published in the Proceedings of the National Academy of Sciences, Stefan Hames and colleagues at the Cornell Lab of Ornithology in Ithaca, New York, found that the wood thrush, a forest bird whose populations have been falling since the 1960s, is less likely to breed in areas that receive high levels of acid-tainted rain, snow, mist and fog. Such regions include the Adirondack, Appalachian and Great Smokey mountains, where wood thrush numbers are dropping by up to 5 percent a year.

The ornithologists made their discovery after analyzing data collected by thousands of volunteers who participate in the lab's Birds in Forested Landscapes project. They hypothesize that the birds' failure to breed may result from acid rain's known ability to leach calcium from soils, which could in turn lead to a shortage of calcium-rich foods such as snail shells, pill bugs and millipedes that are needed to form egg shells and nourish mothers and hatchlings. Acid rain can also lead to elevated levels of toxic aluminum, cadmium and lead in soil. Hames and his colleagues say declines of other North American songbird species might be related to acid rain as well.